One good point you bring up by including senior judges is they can still hear cases and thus contribute to judicial activism.
However most are in their 70’s 80’ and 90’s and don’t hear many cases. Even though they were appointed by Democrats since they are from an earlier era they would be more likely closer to mainstream rather than ultra liberal as well.
While they have an influence based on the cases they hear their rulings would be subject to the full district court re hearing their cases if the ruling were appealed as well as recourse to the appeals court. That is why while we have a lot of work to do the order of importance is from top down, Supreme, Appeals, District. The district court judges seem to retire earlier and President Trump may have the opportunity to appoint many of these positions in the next 6 years.
Speaking of the 3rd circuit, there’s an appeal going through challenging the NJ gun magazine ban that makes anything over 10 rounds illegal. Don’t know what chance it has in the 3rd circuit but if unsuccessful, there was a conflicting ruling in the 9th circuit so hopefully SCOTUS would take it up since it’s effecting the rights of citizens in an entire state. But hopefully Trump can get those vacancies in the 3rd filled quickly, we need all the help we can get.